Examples of microwaves are commercial signals transmitted from satellites and signals of millimetre wavelengths. For the reception of such signals conventionally most often reflection devices are used, such as parabolic antennas, together with a receiver unit.
Waveguide lenses are used for focusing electromagnetic energy to a microwave head in the same way as the parabolic reflectors do. Waveguide lenses consist of a number of short waveguide elements having metal surfaces arranged side by side and having lengths and locations such that the front and rear surfaces of the lens form the contour of a lens, which also can be called a lens profile or a lens shape.
Waveguide lenses made of metal elements are generally very light apparatus. They may thus in many cases be preferred to lenses of dielectric materials.
There are waveguide lenses in a number of configurations in regard of the actual profile of the lens. In a variant, the zone divided or zoned lens, the composition of the waveguides is such that for instance one surface is flat and the other one is rotationally symmetric about the axis of the lens and has an ellipsoidic profile for forming a concave lens. A downstepping of the lens thickness in circular steps of one wavelength may exist for reducing the thickness or depth of the lens at the periphery thereof.
In another embodiment, lenses having a constant thickness, the sizes of the interspaces between parallel metal plates vary, which form the waveguide channels. Such lenses have up to now only been made for a single polarization plane.
In a third case the lens consists of phase shifters in each waveguide element in order to produce the correct focusing of an incident flat electromagnetic wave. These phase shifters cooperate in various structures to make the lens more or less independent of frequency.
The principal advantage of a waveguide lens to a parabolic reflector is that a feeding horn/receiver horn not necessarily has to be arranged and shadow the plane incident wave. Another advantage is that the lens is transparent and open in order to let the wind pass. In particular for large parabolic antennas the problem exists that they at the same time form a large surface exposed to the wind, which is eliminated with an open waveguide lens.
Another advantage of the waveguide lens is the better radiation lobe characteristics thereof and that also an obliquely incident wave front has a distinct focus.
The disadvantage of many of the existing lens types is that they only receive waves having a predetermined polarization, which has to be adapted to the orientation of the lens plates and the phase shifting elements. It makes the reception of satellite signals more difficult, in the case where the satellite signal is polarized in relation to the earth axis, which only agrees with the earth plane in places at the same latitude as the satellite. Everywhere else the polarization of the satellite has a slope in relation to the receiver.
Further, each plate in existing lenses have a very complicated curved profile shape, which in addition is different for plates located at different distances from the lens axis. The positioning of the plates is in addition very critical and requires stable plates, this causing that such a lens will be heavy.
Waveguide lenses have, as has been mentioned above, been proposed having various geometries and structures. They may be constructed of parallel conducting plates having varying profile heights, so that channels of different lengths or depths are formed between the plates, see U.S. Pat. Nos. 2,736,894, 2,785,397, 4,194,209, FR-A1 2 538 959. The channels can also have different widths or be given differently sized impedances in various ways, see for instance U.S. Pat. Nos. 4,321,604, 2,841,793, SU-A1 1589342. The exterior contour of the lenses as generally viewed can be rotationally symmetric having steps or steppings occurring when passing outwards from the geometric axis of the lens, see for instance the mentioned U.S. Pat. No. 2,736,894. Also, entirely rotationally symmetric lenses of dielectric material have been suggested, see U.S. Pat. Nos. 2,705,753, 4,804,970, GB-A 2 155 699. However, up to now the lenses have had a complex construction and have not led to a wide commercial use.